The present invention relates to a rubber composition for a tire tread. More specifically it relates to a rubber composition for a tire tread comprising a diene rubber having, uniformly dispersed therein, a gas-encapsulated thermoplastic resin particle obtained by expanding upon heating at a temperature of an expansion starting temperature or more, prior to vulcanizing a thermally expansible thermoplastic resin particle containing therein a liquid or solid substance capable of generating a gas by vaporization, decomposition or chemical reaction upon heating, followed by vulcanization, said thermally expansible thermoplastic resin particle comprising a thermoplastic resin having an expansion starting temperature of from 70xc2x0 C. to less than 120xc2x0 C. and having a heat resistance to withstand vulcanization of rubber, or a rubber composition for a tire tread further comprising a short fiber, a hard particle and/or a liquid polymer as well as a pneumatic tire having a high frictional force on ice and using the above composition in the tread portion thereof.
In order to increase the friction force on ice of a pneumatic tire, various proposals have been made heretobefore to formulate a hard foreign substance or a hollow particle into a rubber to thereby form a micro irregularity on the surface of a rubber layer, whereby a water-film present on the surface of the ice is removed during a running of a tire and the friction on the ice is increased, but practically satisfactory results have not been obtained yet. Examples of such proposals are disclosed in, for example, JP-A(Kokai)-60-258235 (i.e., micropowder of ceramic), JP-A-2-274740 (i.e., finely divided plant), and JP-A-2-281052 (i.e., metal). However, according to these proposals, there are caused other problems such as the increase in the hardness of the rubber and the loss of flexibility of the rubber whereby the resultant tire is difficult to follow the contours of the road. Typical examples of formulating hollow particles are disclosed in JP-A-2-170840, JP-A-2-208336 and JP-A-4-5543, but there are problems in these arts that the hardness of the rubber is increased, as mentioned above or the hollow particles are broken during the mixing thereof.
Furthermore, although the application examples are different, JP-A-4-246440 is intended to decrease the weight by stably expanding, during vulcanization, a thermally expansible microcapsule compounded in rubber. However, for the purpose of improving the friction force on ice of a tire tread, since the thermally extensible microcapsule cannot be stably expanded during vulcanization when highly reinforcing filler is compounded for satisfying the requirement of high strength in the case of, for example, a pneumatic tire, it is not expected to form the intended irregularity on the surface of a tread sufficient to improve the friction force on ice.
Under the above-mentioned circumstance, the present inventors previously developed a rubber composition for a tire tread having the improved friction force on ice by compounding elastic gas-encapsulated thermoplastic particle having an average particle size of 5-300 xcexcm, to a diene rubber. Since this gas-encapsulated thermoplastic resin compounded rubber forms irregularity on the surface of a tire tread, has a function capable of effectively removing the water-film and maintain the flexibility of rubber, without increasing the hardness of rubber, the pneumatic tire having a high friction force on ice, since the elastic gas-encapsulated thermoplastic particle is compounded.
However, although these proposals exhibit high friction force on ice, further improvements in the friction force on ice are desired.
Accordingly, the object of the present invention is to provide a rubber composition for a tire tread having a further increased frictional force on ice and a pneumatic tire using the same.
Another object of the present invention is to provide a rubber composition for a tire tread having a further increased frictional force on ice and capable of preventing the large deformation of a block at a tire tread portion and a pneumatic tire using the same.
A further object of the present invention is to solve all the above-mentioned problems in the prior art and to provide a rubber composition for a tire tread having a light weight and having a flexibility of rubber per se, without increasing the hardness of a rubber and having a remarkably increased friction force on ice by the scratching effect against ice to remove the water-film on the ice and to provide a spike effect on the ice, when used as a pneumatic tire, and also a pneumatic tire using the same.
A still further object of the present invention is to provide a rubber composition for a tire tread, in addition to the improved frictional force on ice, capable of suppressing the migration of a softening agent and of maintaining a high friction force on ice for a long term and a pneumatic tire using the same.
In accordance with the present invention, there is provided a rubber composition for a tire tread comprising:
100 parts by weight of a diene rubber,
30-90 parts by weight of at least one reinforcing filler selected from the group consisting of carbon blacks and silicas, and
a gas-encapsulated thermoplastic resin particle having an average particle size of 30-150 xcexcm obtained by expanding 1-20 parts by weight of a thermally expansible thermoplastic resin particle containing therein a liquid or solid substance capable of generating a gas by vaporization, decomposition or chemical reaction upon heating, upon heating at a temperature of an expansion starting temperature or more, prior to the vulcanization of rubber, followed by vulcanization, said thermally expansible thermoplastic resin particle comprising a thermoplastic resin having an expansion starting temperature of from 70xc2x0 C. to less than 120xc2x0 C. and having a heat resistance to withstand the vulcanization of a rubber, and a pneumatic tire using the same as a tread portion.
In accordance with the present invention, there are also provided a rubber composition for a tire tread as mentioned above 1 to 20 parts by weight of at least one member selected from the group consisting of a short fiber and a hard particle having a Vickers hardness of 35-1000 and an average particle size of 20 to 500 xcexcm further compounded and a pneumatic tire using the same as a tread portion.
In accordance with the present invention, there is provided a process for producing a rubber composition for a tire tread comprising:
after, or while, uniformly mixing a diene rubber with a thermally expansible thermoplastic resin particle having an expansion starting temperature of from 70xc2x0 C. to less than 120xc2x0 C. and having a heat resistance to withstand vulcanization of rubber and containing an encapsulated liquid or solid substance capable of generating a gas by evaporation, decomposition or chemical reaction upon heating, followed by heating at a temperature of the expansion starting temperature of the thermally expansible particle or more to expand the particle, and
vulcanizing the expanded particle to uniformly disperse the gas-encapsulated thermoplastic resin particle having an average particle size of 30-150 xcexcm in the rubber.
In accordance with the present invention, there is further provided a rubber composition for a tire tread comprising:
100 parts by weight of a diene rubber;
30-90 parts by weight of at least one reinforcing filler selected from the group consisting of carbon blacks and silicas;
a gas-encapsulated thermoplastic resin particle obtained by expanding, upon heating at a temperature of an expansion starting temperature or more, prior to vulcanization, 1-20 parts by weight of a thermally expansible thermoplastic resin particle containing therein a liquid or solid substance capable of generating a gas by vaporization, decomposition or chemical reaction upon heating, followed by vulcanization, said thermally expansible thermoplastic resin particle comprising a thermoplastic resin having an expansion starting temperature of from 70xc2x0 C. to less than 120xc2x0 C. and having a heat resistance to withstand vulcanization of rubber; and
5-55 parts by weight of a liquid polymer having a weight-average molecular weight of 6,000-100,000 and a glass transition temperature (Tg) of xe2x88x9250xc2x0 C. or less.